Supported Pd-alloy membranes are used for the separation of hydrogen (H2) from gas mixtures. The dense Pd-alloy (PdA) structure only allows the H2 to pass in the form of interstitial H atoms. As a result, it blocks all other gases. PdA membranes generally operate at temperatures above 300° C. Pd is the major constituent. The Pd is typically alloyed with other metals, including, but not limited to, Cu, Ag, and Au. Alloying Pd with other metals has several purposes. First, alloying helps to suppress a destructive α-β phase transformation. In addition, it makes the membranes more stable against contaminants in practical processing atmospheres.
H2 separation with PdA membranes has the ability to produce very pure H2 for use in Polymer Electrolyte Membrane (PEM, also called Proton Exchange Membrane) fuel cells. H2 produced using PdA membranes should contain less than 20 ppm CO. PdA membranes could be used in membrane reactors with H2 semi-permeable walls. Selective removal of H2 in reforming or coal gasification mixtures could result in significant conversion enhancement and greatly improved space yield.
Consequently, PdA membranes are being considered for use in new coal-fired power plants. State of the art PdA membranes are made as either stand-alone thin-walled tubes, or supported on a multi-layer porous carrier structure. The thin walled tubes have a thickness of about 20 μm, while the supported PdA membranes have a thickness of about 3 μm or more. The supported PdA membrane concept is preferred for reasons of cost-price, H2 production performance, and mechanical stability and strength.
Currently, development is being done with a) unsupported PdA tubes more than 10 μm thick, b) with stainless steel carriers provided with a Tosoh zirconia layer, and about 3 μm or more thick PdA by electroless deposition, and c) with stainless steel carriers with very high surface roughness provided with a ceramic buffer layer by oxidation and more than 3 μm thick PdA by electro-deposition technique.
The use of state of the art supported PdA membranes involves major challenges. One problem is developing a PdA membrane that is sufficiently thin, preferably less than 1 μm, and which is free of pinholes, which has sufficient adhesion to the support layer, and which is stable at the operating conditions. Another problem is developing a supported PdA membrane structure with a membrane module area/volume greater than 100 m−1, and with a cost/price of less than $500/m2.
Part of the technical problems with the state of the art PdA membranes relates to the generally inferior homogeneity of the supporting structure which results in an irregular deposition surface, poor adhesion, and instability during the life-time of the membrane. The cost/price of the state of the art supported membranes does not depend as much on the PdA raw material price, as on the cost/price of the supporting structure, the manufacturing time and temperature of the supporting structure, and the currently inferior reproducibility and stability of supported membrane structures.
Therefore, there is a need for a thin, pinhole-free PdA membrane which is cost effective.